The present invention is directed toward an improved instrument for use in providing an accurate measurement of the intraocular pressure (IOP) of an eye without making any physical contact with the eye and without need for eye drops or anesthetic. The instrument of the present invention achieves these measurements through non-invasive and non-contact techniques, thereby providing an improved method for use in the early detection of glaucoma.
Glaucoma is an eye disease which can damage the optic nerve and which is one of the leading causes of blindness in the U.S. and throughout the world. Two out of every one hundred persons over age 35 have vision threatened by glaucoma.
When an object is viewed, the image is carried from the retina of the eye to the brain by the optic nerve. The optic nerve is an accumulation of over one million individual transmitters, each carrying a message to the brain. The individual messages all join together to provide side vision or peripheral vision as well as sharp central vision. Glaucoma can permanently damage the optic nerve, causing blind spots in areas of vision to develop. If glaucoma is undiagnosed, the optic nerve may sustain considerable irreversible damage and may even be destroyed, resulting in blindness.
Glaucoma is detectable by measuring the intraocular fluid pressure at the front surface or cornea of the eye. Intraocular fluid flows through the inner eye continuously to maintain the structure of the eye, in particular, the cornea. If the outflow or drainage system within the eye becomes blocked for any reason, the fluid backs up within the inner eye causing the intraocular fluid pressure to increase, thereby increasing the potential for damage to the optic nerve. The primary preventative measure which can be taken is the early detection of glaucoma by periodic testing of the intraocular pressure (IOP) since an elevated intraocular pressure (IOP) is clearly basic to the whole concept of glaucoma.
A variety of devices have been devised to facilitate the measurements of the intraocular pressure. The most common is a tonometer which measures the force necessary to applanate or flatten a given area of the cornea. An adjustable known force is applied to flatten a predetermined area of the cornea. This permits a direct measure of pressure to be made because the force and the area are directly known. The most common unit of this type is the Goldmann tonometer. While accurate in its measurements, the Goldman tonometer is an undesirable tool for many reasons. It is designed to provide a single time-segregated measurement of intraocular pressure. The application-type tonometer must be used with a topical anesthetic and a fluorescein dye. It includes physical touching of the eye, which many patients find objectionable. There is also an inherent risk of abrasion, injury, or infection to the eye as a result of contact.
Another common tonometer apparatus is the Schiotz or plunger-type tonometer. The Schiotz tonometer is placed before the eye along the optical axis and a plunger is released which flattens the cornea to a specified diameter and measures the forces applied. The Schiotz tonometer has the same undesirable qualities as the Goldman tonometer. It has been found that the patient usually has a somewhat high level of fear and physical discomfort as a result of such eye contact. Thus, the patient will tend to avoid the procedure, if possible.
A new generation of tonometers have been designed in an effort to limit physical contact with the eye which utilize a very strong air puff that impacts the eye. The air puff impinges on the cornea causing a sudden curvature reduction, applanation, and finally a slight concavity before restoration. Patient objections are still encountered when using the air puff system due to the discomfort caused by the force of the air puff on the eye and the accompanying audible explosion of the air puff as it is generated. Other disadvantages include the fact that an air puff measurement is a one-time occurrence and may, therefore, be offset from the actual average pressure value.
Other types of non-contact tonometers are disclosed in U.S. Pat. Nos. 4,928,697 and 5,148,807 both of which are assigned to the assignee of the present invention. The tonometers disclosed in those patents utilize the principles of induced phase modulation and/or frequency modulation of optical or acoustic waves which are directed toward the cornea as a diagnostic beam. The high frequency diagnostic waves are transmitted either as high frequency sonic waves or visible or invisible light waves.
Another non-contact instrument for measuring displacement of the cornea is disclosed in a thesis of Theodore Trost entitled Laser Interferometer Having Multiple Sensors which was published in 1995 and is available at The Ohio State University Library. As disclosed in that thesis, there is provided an interferometric displacement measurement apparatus having a coherent laser beam incident upon a partially reflective mirror, forming a measurement beam which is reflected back onto a sensor field. The incident beam also forms a reference beam incident upon the sensor field. The sensor field comprises at least two and preferably three or more photodetecting sensors spaced radially of the measurement beam axis arriving at the sensor field.
The interferometer disclosed is theoretically designed to measure the relative displacement of a target surface such as the surface of the cornea.
The Trost interferometer has many deficiencies and was not successfully reduced to practice. Problems were encountered in translating the relative displacement of the surface of the cornea to provide meaningful measure of intraocular pressure. There is ambiguity in determining the absolute direction of movement of the cornea surface and problems in eliminating excessive ambient noise received and measured by the system. Finally, measurements taken by the Trost apparatus are found to bear no statistical relationship to like measurements taken by a Goldman apparatus.
Most recently, an interferometer utilizing optical modulation to measure optical displacement has been patented to Gust (U.S. Pat. No. 5,828,454). Gust teaches the measurement of the static and dynamic displacement of a cornea by measuring the phase shift of an optical pathway. While the Gust patent is predicated on the theory that the measured phase shift is linearly proportional to deflection of the cornea, recent research has established that such a direct correlation is not necessarily as simple and accurate as Gust presents.
For instance, the eye has a multiplicity of reflective surfaces such as the lens, iris, front surface of the tear layer and the corneal surface. If the optical beam is not properly focused, it cannot be accurately predicted which surface is reflecting the beam, thus reducing the dependability and reliability of the instrument.
These and other non-contact tonometer attempts to make use of light waves and sound waves to measure corneal displacement have all suffered from two major deficiencies: the inability to accurately focus the measurement beam on the cornea and align the measurement beam with the sensor. Many complicated physical and mathematical techniques designed to meet and overcome these techniques have contravened the goal of simplicity in obtaining an accurate measurement of intraocular pressure by means of a non-contract tonometer.
Therefore, it is an object of the invention to provide an accurate non-invasive method and apparatus for performing the method of measuring the intraocular pressure of an eye.
A further object of the invention is to provide a method and apparatus for performing the measurement of the intraocular pressure of an eye continuously for a selected period of time in order to view variations in the pressure over time.
Yet another object of the present invention is to apply diagnostic energy to the cornea in a controlled, non-invasive, direct manner to accurately focus the energy onto a desired surface of the eye.
Yet another object of the invention is to provide a non-contact tonometer that can be adopted for use in an office or hospital on a fixed stand or, alternatively, be provided as a portable unit for use by health care professionals working, for instance, in nursing homes and assisted living homes, or be provided as a portable home unit simple enough to be used by individuals with little or no health care training.
It is a final object of the present invention to provide an instrument which accurately measures the intraocular pressure through non-invasive and non-contact type techniques.